Metal oxide electrode for supercapacitor and manufacturing method thereof

ABSTRACT

A metal oxide electrode for a supercapacitor and a manufacturing method thereof are disclosed. Potassium permanganate is absorbed on a conductive material, such as carbon or activated carbon, and mixed with a solution including manganese acetate so as to form amorphous manganese oxide. Amorphous manganese oxide powder is grounded to a powder which is mixed with binder to form an electrode having a predetermined shape. The electrode reduces equivalent serial resistance and enhances high frequency characteristics since the contact area and the adhesion strength between the manganese oxide and the conductive carbon are improved. Also, the electrode has high capacitance suitable for a supercapacitor, which is manufactured therefrom at a greatly reduced cost.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a metal oxide electrode for asupercapacitor and a method for manufacturing the same, and moreparticularly to a metal oxide electrode for a supercapacitor includingmanganese oxide as an active material of the electrode, and a method formanufacturing the same.

2. Description of the Related Art

The recent advancement of scientific civilization accelerates the use ofvarious high technology electronic devices. These devices are essentialfor modern life, but such devices produce many environmental problems,such as increasing waste and pollution. Considering those problems, agreat deal of effort has been expended to develop an alternative energystoring device having high capacity and long durability withoutpollution. Also, the need for memory devices which can convenientlycontrol various electronic devices has rapidly increased.

Because of the problem that most electronic devices are subject tomemory loss, thus causing errors, when an undesired stoppage or even avariation of power occurs, the need for memory back up power continuallyincreases. To meet such need, much research has been undertaken. One ofthe best solutions of recent research is development of anelectrochemical capacitor, which is called a supercapacitor. It has agreatly enhanced storage capacitance which is more than hundreds tothousands of times larger than that of conventional capacitors. Also,the supercapacitor has high energy density and excellent power densitywhich is hundreds of times more than the power density of a battery,thereby providing much stable and powerful energy to electronic devices.

The electrochemical capacitor is generally divided into threecategories, such as an Electric Double Layer Capacitor (ELDC), a metaloxide pseudocapacitor and a conducting polymer capacitor, depending onthe energy storage mechanism and active materials used in each system.In the metal oxide pseudocapacitor, the active material is generallyconductive metal oxide which has high surface area and electrochemicalreactivity with working ions in an electrolyte. Electrochemicalreduction and oxidation reactions as well as physical charge separationbetween an electrode and electrolyte interface, are the energy storagemechanisms. On the other hand, the ELDC uses activated carbon with alarge surface area as an active material, and physical charge separationbetween an electrode and an electrolyte interface is the main energystorage mechanism.

It is generally appreciated that the metal oxide pseudocapacitor canobtain higher capacitance than the EDLC because, as mentioned above, themetal oxide pseudocapacitor can get its capacitance using anelectrochemical redox reaction as well as the physical charge separationat the electrolyte interface, whereas the EDLC system only obtains itscapacitance from the physical charge separation at the electrolyteinterface.

The supercapacitor generally consists of porous active materialelectrodes, a separator, an electrolyte, a current collector, a case andterminals. The current collector can be composed of high electricalconductivity material, such as metal or a conducting film. The case andthe terminals should be composed of light materials to reduce the weightof the capacitor. The separator and the electrolyte relate to the ionicconductivity of the capacitor. The current collector and the terminalsare concerned with electrical conductivity of the capacitor. Theelectrical and the ionic conductivities are the main factors indetermining the output characteristics of the capacitor.

The manganese oxide with layered structure can be a candidate for anelectrode of the metal oxide pseudocapacitor, which has been studied asan electrode of rechargeable batteries. The manganese oxide, includinglayered structure having potassium ions therein, is obtained bythermally decomposing potassium permanganate or chemical reactions.

The reaction of the supercapacitor is a surface reaction, while thereaction of the manganese oxide is an intercalation reaction. Thus, suchmanganese oxide may not be applied to the electrode of thesupercapacitor because the supercapacitor has a rapidlycharging/discharging, wide permissible temperature range and highelectrical conductivity. But, depending on a condition of materialsynthesis, such as cooling rate, surface condition as a mean valence ofmanganese ion of material can be changed to show good capacitorperformance. Moreover, intercalation reaction also may contribute to thetotal capacitance of the material in special capacitor operation, suchas slow charge and discharge conditions.

Ruthenium oxide (RuO₂) has recently been utilized as an electrode of acapacitor. The capacitor using ruthenium oxide as an electrode has ahigh capacitance of about 700 F/g, which is much higher than that ofconventional capacitors. However, ruthenium oxide is too expensive toapply to the capacitor electrodes, i.e., the manufacturing cost of aruthenium oxide electrode is hundreds of times higher than that of aconventional electrode. Furthermore, the high capacitance of rutheniumoxide can be obtained only when an acid solution, such as that ofsulfuric acid (H₂SO₄), is used therewith, which causes a seriousenvironmental hazard.

The present inventors reported that amorphous manganese oxide has goodproperties as an electrode for a supercapacitor in a neutralelectrolyte, such as potassium chloride (refer to Journal of Solid StateChemistry, vol. 144, pages 220 to 223, 1999, entitled “SUPERCAPACITORBEHAVIOR WITH KCl ELECTROLYTE”). However, when amorphous manganese oxideis directly used as the electrode of a supercapacitor, the EquivalentSerial Resistance (ESR) of the supercapacitor may be greatly increasedwhen operating at a high frequency, and the energy loss of thesupercapacitor may be seriously increased at a low frequency becauseamorphous manganese oxide has low conductivity at room temperature.

Although the electrode for the supercapacitor is manufactured byphysically mixing conductive carbon having good electrical conductivitywith amorphous manganese oxide, the increasing capacitance of thesupercapacitor and the reducing volume of the supercapacitor may belimited since little manganese oxide can be included in conductivecarbon by specific volume. Also, the physical mixing process has somedisadvantages; the contact area between the manganese oxide and theconductive carbon is reduced, and the degree of dispersion of themanganese is limited.

SUMMARY OF THE INVENTION

Considering the above-described problems and disadvantages, it is anobject of the present invention to provide a metal oxide electrode for asupercapacitor having high capacitance.

It is another object of the present invention to provide a method formanufacturing a metal oxide electrode for a supercapacitor having highcapacitance.

It is still another object of the present invention to provide a metaloxide electrode for a supercapacitor having a low ESR and an enhancedhigh frequency characteristic in a neutral electrolyte.

It is still another object of the present invention to provide a methodfor manufacturing a metal oxide electrode for a supercapacitor having alow ESR and an enhanced high frequency characteristic in a neutralelectrolyte.

To achieve these objects, the present invention provides a metal oxideelectrode comprising manganese oxide powder, conductive material andbinder.

Preferably, the binder is composed of or comprisespolytetrafluoroethylene.

According to one embodiment of the present invention, the conductivematerial is conductive carbon and the manganese oxide powder is coatedon the conductive carbon. As for the present invention, many kinds ofhighly conductive materials can replace the conductive carbon becausethe roles of the conductive carbon of the present invention are makingan electrical conduction path and sites of the amorphous manganese oxidecoating. Therefore, basically all conductive materials, such as metaloxide, metal nitride, metal carbide, metal powder and conductingpolymer, are suitable for this purpose.

Preferably, the electrode comprises from approximately 20 to 80% byweight of conductive carbon.

According to the another embodiment of the present, the conductivematerial is an activated carbon and the manganese oxide powder is coatedin pores of the activated carbon and on the surface of the activatedcarbon.

Preferably, the electrode comprises from approximately 20 to 80% byweight of the activated carbon. In this case, the activated carbonpreferably has a specific surface area of from approximately 1500 to3000 m²/g.

In another embodiment of the present invention, the conductive materialis also a conductive carbon and the manganese oxide powder is coated onthe conductive carbon. In this case, the electrode preferably includesfrom approximately 30 to 90% by weight of the manganese oxide powder,from approximately 5 to 50% by weight of the conductive carbon and fromapproximately 5 to 50% by weight of binder.

Also, to accomplish the above objects of the present invention, there isprovided a method for manufacturing a metal oxide electrode for asupercapacitor comprising steps of: forming a conductive materialsolution by dispersing a conductive material in deionized water; forminga first solution by adding potassium permanganate to the conductivematerial solution; forming a second solution comprising manganeseacetate; forming amorphous manganese oxide by mixing the first solutionwith the second solution; and forming the metal oxide electrodeincluding the amorphous manganese oxide.

According to one embodiment of the present invention, the step offorming the conductive material solution is performed after dissolving asurfactant in the deionized water. At that time, the surfactant ispreferably composed of polyvinylpyrrolidone.

In another embodiment of the present invention, the conductive materialis conductive carbon or activated carbon, and the first solution is apotassium permanganate solution, wherein the potassium permanganate isabsorbed into the conductive material.

Preferably, the step of forming the metal oxide electrode includessubsteps of extracting amorphous manganese oxide powder from the firstsolution and the second solution, grinding the amorphous manganese oxidepowder, forming a mixture by adding binder to the ground amorphousmanganese oxide powder and forming an electrode having a predeterminedshape by using the mixture.

The step of extracting the amorphous manganese oxide powder preferablyincludes substeps of filtering the amorphous manganese oxide powder froma mixture of the first solution and the second solution, washing thefiltered amorphous manganese oxide powder and drying the washedamorphous manganese oxide powder. In this case, the binder is composedof polytetrafluoroethylene.

According to another embodiment of the present invention, there isprovided a method for manufacturing a metal oxide electrode for asupercapacitor comprising steps of grinding potassium permanganate;heating a firnace at a first predetermined temperature; thermallydecomposing the ground potassium permanganate in the furnace; quenchingthe product to a second predetermined temperature; washing and filteringthe product; and mixing the product with conductive material, binder andsolution.

Preferably, the furnace is heated to a temperature of from approximately450 to 550° C. and the step of thermally decomposing the groundpotassium permanganate is performed to form manganese oxide having alayer structure and comprising potassium ions therein.

The step of quenching the product is performed by rapidly cooling theproduct to a temperature below room temperature. In this case, theconductive material is conductive carbon and the binder is composed ofpolytetrafluoroethylene.

In one method for manufacturing a metal oxide electrode for asupercapacitor according to a preferred embodiment of the presentinvention, a surfactant is sufficiently dissolved in deionized water,and then conductive carbon is dispersed in the deionized water(including the surfactant) to form a conductive carbon solution. Afterthe conductive carbon is sufficiently dispersed in the conductive carbonsolution, potassium permanganate (KMnO₄) is added to the conductivecarbon solution and is absorbed on the surface of the conductive carbon,thereby forming a potassium permanganate solution. Amorphous manganeseoxide (MO₂.nH₂O) is prepared by mixing the potassium permanganatesolution with a manganese acetate solution. Amorphous manganese oxidepowder is extracted from the mixed solution through filtering, washingand drying processes. The amorphous manganese oxide powder is ground andmixed with binder to form a mixture. Then, the mixture is formed tomanufacture an electrode for a supercapacitor having a predeterminedshape. In this case, the electrode comprises from approximately 20 to 80weight percent (wt%) of the conductive carbon.

As for another method for manufacturing a metal oxide electrode for asupercapacitor according to another preferred embodiment of the presentinvention, an activated carbon powder is sufficiently dispersed indeionized water to form an activated carbon solution, and then potassiumpermanganate is added to the activated carbon solution, thereby formingpotassium permanganate solution including the potassium permanganateabsorbed in pores of the activated carbon and on the surface of theactivated carbon. An electrode for a supercapacitor is manufacturedaccording to the above described method after amorphous manganese oxideis formed by mixing the potassium permanganate solution with a manganeseacetate solution. In this case, the activated carbon has a specificsurface area of from approximately 1500 to 3000 m²/g. The electrodecomprises from approximately 20 to 80 wt % of the activated carbon.Preferably, the activated carbon has a specific surface area ofapproximately 2000 m²/g, and the electrode is composed of approximately40 wt % of the activated carbon.

In another method for manufacturing a metal oxide electrode for asupercapacitor according to a further preferred embodiment of thepresent invention, after potassium permanganate is inserted into afurnace previously heated to a predetermined temperature, the potassiumpermanganate is thermally decomposed in the furnace, and then rapidlycooled below room temperature to form a manganese oxide powder. Suchmanganese oxide powder has stable chemical structure and compositionsince the crystal and the particle growths in the manganese oxide powdercan be limited during slow heating and slow cooling processes. Thethermal decomposition reaction of the potassium permanganate proceeds asthe following expression.

10K₂MnO₄→2.6₅K₂MnO₄+(2.3₅K₂,7.3₅MnO₂.O₅)+6O₂

wherein the subscripts 5 mean significant figures.

The temperature of the furnace is from approximately 450 to 550° C.during the thermal decomposition reaction. Preferably, the thermaldecomposition reaction proceeds at temperature of approximately 500° C.

If the thermal decomposition reaction occurs below 450° C., manganeseoxide cannot be obtained since the thermal decomposition reaction beginsonly above 450° C. Also, surface conditions of the manganese oxide,including mean valence of manganese ion, are not adequate to yield goodcapacitance when the thermal decomposition reaction occurs above 650° C.

FIG. 1 is a graph showing X-ray diffraction analysis of the manganeseoxide powder according to a preferred embodiment of the presentinvention. Such manganese oxide is manufactured by washing the thermallydecomposed potassium permanganate with distilled water and drying it at120° C. after the potassium permanganate is thermally treated atapproximately 500° C. for approximately 2 hours and rapidly cooled toroom temperature.

Referring to FIG. 1, the manganese oxide powder of the present inventionhas nearly a delta (δ) phase as the layered structure.

FIG. 2 is a graph showing particle size distribution of the manganeseoxide powder in FIG. 1. As shown in FIG. 2, most of the particles of themanganese oxide powder have diameters of from approximately 0.1 to 1.0μm.

FIG. 3 is a graph illustrating measured capacitances of manganese oxidepowders respectively prepared at various temperatures according toanother preferred embodiments, and

FIG. 4 is a graph illustrating unit capacitances of the manganese oxidepowders respectively manufactured for various reaction times accordingto another preferred embodiments. In FIG. 4, the thermal decompositionreactions occur at approximately 500° C.

Referring to FIGS. 3 and 4, the manganese oxide powder manufactured at500° C. for 2 hours has the highest specific capacitance.

When the manganese oxide having layered structure is applied to theelectrode for the supercapacitor, the conductive material, such as theconductive carbon, is mixed with the manganese oxide since the manganeseoxide having the layered structure has relatively low electricalconductivity. Otherwise, the energy of the capacitor may reduce when thecapacitor is operated at low frequency. Also, the binder is added to theconductive material having the manganese oxide coated thereon forcoating the mixture on a current collector to have a film shape. If theelectrode comprises below 30 wt % of the manganese oxide or above 80 wt% of the conductive material, the electrode cannot have propercharacteristics for the supercapacitor. When the electrode comprisesabove 90 wt % of the manganese oxide or below 5 wt % of the conductivematerial, the ESR of the electrode increases, so the capacitor havingsuch an electrode may operate only with difficulty at a high frequencyor at a low frequency. In addition, the mixture cannot be coated on thecurrent collector when the electrode comprises below 5 wt % of thebinder and the electrode may not be applied to the supercapacitor whenthe electrode includes above 50 wt % of the binder. The electrode of thepresent invention has a high capacitance of approximately 300 F/g incase of a water-soluble electrolyte.

According to the present invention, the electrode for the supercapacitorcan reduce the ESR and enhance high frequency characteristics since thecontact area and the adhesion strength between the manganese oxide andthe conductive carbon are improved.

Also, the electrode of the present invention has high capacitancesuitable for a supercapacitor as compared with conventional electrodesbecause the electrode is manufactured by mixing amorphous manganeseoxide powder with the conductive material and the binder.

Furthermore, the electrode of the present invention can be manufacturedat a cost of one percent of that of the ruthenium oxide electrode, eventhough the electrode of the present invention has good capacitancenearly equal to half the capacitance of the ruthenium oxide electrodewhich has the highest capacitance among conventional electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

The above object and other advantages of the present invention willbecome more apparent by describing in detail preferred embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is a graph showing X-ray diffraction analysis of manganese oxideaccording to a preferred embodiment of the present invention;

FIG. 2 is a graph showing particle distribution of manganese oxide inFIG. 1;

FIG. 3 is a graph illustrating measured capacitances of manganese oxidepowders respectively prepared at various temperatures according apreferred embodiment of the present invention;

FIG. 4 is a graph illustrating specific capacitances of manganese oxidepowders respectively manufactured for various reaction times according apreferred embodiment of the present invention;

FIG. 5A to FIG. 5F are graphs showing cyclic voltammograms (CV) of anelectrode for a supercapacitor measured by changing voltage scanningspeeds according to the first experiment of the present invention;

FIG. 6A to FIG. 6D are graphs showing cyclic voltammograms of anelectrode for a supercapacitor measured by changing voltage scanningspeeds according to the second experiment of the present invention;

FIG. 7A to FIG. 7F are graphs showing cyclic voltammograms of anelectrode for a supercapacitor measured by changing voltage scanningspeeds according to the third experiment of the present invention;

FIG. 8 is a graph illustrating variations of specific capacitances ofthe electrodes and amount of the conductive carbon in the electrodes ata constant voltage scanning speed of 20 mV/sec according to the first,second and third experiments of the present invention;

FIG. 9 is a graph illustrating variations of specific capacitances ofthe conventional electrode and an electrode of the first experiment ofthe present invention;

FIG. 10 is a graph illustrating the cyclic voltammograms of theconventional electrodes and the electrodes of the first experiment ofthe present invention at a constant voltage speed of 20 mV/sec;

FIG. 11 is a graph illustrating a test result of the electrode of thefirst experiment as the electrode for the supercapacitor;

FIG. 12 is a graph showing the X-ray diffraction analysis of amorphousmanganese oxide according to the fifth experiment of the presentinvention,

FIG. 13 is a graph illustrating cyclic voltammogram of the electrode forthe supercapacitor according to the fifth experiment of the presentinvention;

FIG. 14A to FIG. 14F are graphs showing cyclic voltammograms of theelectrode for the supercapacitor according to the fifth experiment ofthe present invention;

FIG. 15 is a graph illustrating variation of unit capacitance of theelectrode for the supercapacitor according to the fifth experiment ofthe present invention;

FIG. 16 is a graph showing specific capacitances of the electrodes forthe supercapacitor according to the fourth, fifth, sixth and seventhexperiments of the present invention;

FIG. 17 is a graph showing specific capacitances of electrodes havingdifferent amount of activated carbon according to the fourth, fifth,sixth and seventh experiments of the present invention;

FIG. 18 is a graph showing an alternating current impedance of a 2Vcapacitor manufactured by using the electrode according to the fifteenthexperiment of the present invention;

FIG. 19 is a graph showing the cyclic voltammogram of the 2V capacitorin FIG. 18;

FIG. 20 is a graph illustrating performance tests for the electrodeaccording to the fifteenth experiment of the present invention; and

FIG. 21A to FIG. 21D are graphs showing cyclic voltammograms of theelectrode according to the fifteenth experiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, various embodiments of the present invention will beexplained in more detail with reference to the accompanying figures,however, it is understood that the present invention should not belimited to the following preferred embodiments.

Experiment 1

After a conductive carbon powder was added to 60 ml of deionized water,the conductive carbon powder was sufficiently dispersed and wetted inthe deionized water by stirring the deionized water, including theconductive carbon, to form conductive carbon solution. In this case, asurfactant was previously dissolved in the deionized water in order toachieve complete dispersion of the conductive carbon powder because theparticles of the conductive carbon powder are hydrophobic. When thesurfactant may not be added to the deionized water, potassiumpermanganate cannot be absorbed in the conductive carbon powder in thesubsequent step since the particles of the conductive powder are notsufficiently wetted by the deionized water.

In the present experiment, the conductive carbon was acetylene blackmanufactured by Chevron Chemical Company in the United States. In thiscase, the amount of the conductive carbon was 20 weight percent (wt%),40 wt %, 60 wt % and 80 wt %, respectively. The amount of the conductivecarbon was calculated on grounds of the total amount of the manganeseoxide powder prepared by mixing potassium permanganate with manganeseacetate during the subsequent steps. The surfactant was 0.06 g ofpolyvinylpyrrolidone (PVP).

Subsequently, permanganate solution was prepared by adding 1.58 g ofpotassium permanganate (KMnO₄) to the conductive carbon solution. Then,the permanganate solution was stirred for 1 hour so that the potassiumpermanganate was sufficiently absorbed on the conductive carbon.

Meanwhile, manganese acetate solution was prepared by adding 3.68 g ofmanganese acetate to 100 ml of deionized water.

The manganese acetate solution was mixed with the permanganate solutionto form a mixed solution, and then the mixed solution was violentlystirred. When the manganese acetate solution is mixed with thepermanganate solution, the formation of amorphous manganese oxide torapidly proceeds so that the color of the mixed solution is quicklychanged to brown, which is the color of the amorphous manganese oxide,and the viscosity of the mixed solution rapidly increases. Thus, themixed solution was stirred for 12 hours in order to execute theformation reaction of the amorphous manganese oxide in the presentexperiment.

After the mixed solution including the amorphous manganese oxide wasfiltered several times by using a ceramic filter, thereby obtainingamorphous manganese oxide powder, the amorphous manganese oxide powderwas washed with deionized water.

Then, the amorphous manganese oxide powder was sufficiently dried in adryer at a temperature of 120° C. after the amorphous manganese oxidepowder was inserted into the dryer.

After the sufficiently dried amorphous manganese oxide powder wasground, a mixture was formed by mixing binder with the ground amorphousmanganese oxide powder. The binder was polytetrafluoroethylene (PTFE).

Subsequently, sheet shaped electrodes were cut to form pellet shapedelectrodes after the sheet shaped electrodes were formed by rolling themixture.

Finally, manganese oxide electrodes for the supercapacitor weremanufactured by cold drawing the pellet shaped electrodes on currentcollectors.

FIG. 5A to FIG. 5F are graphs showing cyclic voltammograms (CV) of theelectrode for the supercapacitor according to the present experiment. InFIGS. 5A to 5F, the cyclic voltammograms of the electrode for thesupercapacitor were measured by changing voltage scanning speeds. Inthis case, the electrode for the supercapacitor includes 40 wt % of theconductive carbon.

Referring to FIGS. 5A to 5F, the electrode for the supercapacitor of thepresent experiment has excellent current responsibility and thecharging/discharging of the electrode for the supercapacitor can beaccomplished by a constant rate concerning the whole cycle.

Experiment 2

As for the present experiment, electrodes for the supercapacitor weremanufactured by the method of experiment 1 except for the amount ofconductive carbon in the electrodes for the supercapacitor.

According to the present experiment, the conductive carbon was SUPER-Pmanufactured by M.M.M. Carbon in Belgium, and the amount of theconductive carbon was 20 wt %, 40 wt %, 60 wt % and 80 wt %,respectively. In this case, the amount of the conductive carbon wasevaluated on grounds of the total amount of the manganese oxide powderprepared by mixing the potassium permanganate with the manganeseacetate.

FIG. 6A to FIG. 6D are graphs showing cyclic voltammograms (CV) of theelectrode for the supercapacitor according to the present experiment. InFIGS. 6A to 6D, the cyclic voltammograms of the electrode for thesupercapacitor were measured by changing voltage scanning speeds, andthe electrode for the supercapacitor includes 60% by weight of theconductive carbon.

As shown in FIGS. 6A to 6D, the electrode for the supercapacitor of thepresent experiment has rapid current response, and thecharging/discharging of the electrode can be accomplished by a constantrate concerning the whole cycles.

Experiment 3

In the present experiment, electrodes for the supercapacitor weremanufactured by the method as that of experiment 1 except for the typeof the conductive carbon and the amount of the conductive carbon in theelectrodes for the supercapacitor.

According to the present experiment, the conductive carbon was KetjenBlack EC manufactured by Lion Corporation in Japan, and the amount ofthe conductive carbon was 20 wt %, 40 wt %, 60 wt % and 80 wt %,respectively.

FIG. 7A to FIG. 7F are graphs showing cyclic voltammograms (CV) of theelectrode for the supercapacitor according to the present experiment. InFIGS. 7A to 7F, the cyclic voltammograms of the electrode for thesupercapacitor were measured by changing voltage scanning speeds and theelectrode for the supercapacitor includes 40% by weight of theconductive carbon.

As shown in FIGS. 7A to 7F, the electrode for the supercapacitor of thepresent experiment has rapid current response, and thecharging/discharging of the electrode can be accomplished by a constantrate concerning the whole cycle.

FIG. 8 is a graph illustrating the variations of specific capacitancesof the electrodes and the amount of the conductive carbon in theelectrodes according to experiments 1, 2 and 3 of the present invention.At that time, the variations of the specific capacitances according tothe type of conductive carbon were measured at constant voltage scanningspeed of 20 mV/sec. The specific capacitance mean standardized valuesare obtained by dividing measured capacitance by weights of themanganese oxide coated on the conductive carbon.

Referring to FIG. 8, the specific capacitances of the electrodes haveexcellent values from approximately 250 to 325 F/g when the conductivecarbon is SUPER-P. Particularly, when the electrodes respectivelyinclude 40 wt % and 60 wt % of the conductive carbon, the specificcapacitances of the electrodes show the best values of approximately 320F/g and 325 F/g.

FIG. 9 is a graph illustrating variations of specific capacitances of aconventional electrode and an electrode of experiment 1 of the presentinvention. The specific capacitances are measured by changing thevoltage scanning speed. In the electrode of experiment 1, the conductivecarbon is SUPER-P, and the electrode includes 60 wt % of the conductivecarbon on which manganese oxide is coated; however, the conventionalelectrode is prepared by physically mixing the conductive carbon withthe manganese oxide.

As shown in FIG. 9, the electrode of experiment 1 has a specificcapacitance higher than that of the conventional electrode.

FIG. 10 is a graph illustrating cyclic voltammograms of conventionalelectrodes and the electrodes of experiment 1 of the present inventionat a constant voltage speed of 20 mV/sec. That is, the electrode ofexperiment 1 includes 60 wt % of SUPER-P as the conductive carbon onwhich the manganese oxide is coated, while the conventional electrode isprepared by physically mixing conductive carbon with manganese oxide.

Referring to FIG. 10, the electrode of the experiment 1 has high currentresponse at both end points of the voltage. Hence, the electrode ofexperiment 1 is greatly suitable for a supercapacitor.

FIG. 11 is a graph illustrating a test result of the electrode ofexperiment 1 as the electrode for the supercapacitor. The electrodecomprises 60 wt % of SUPER-P as the conductive carbon on which manganeseoxide is coated.

Referring to FIG. 11, the electrode of the present invention showsexcellent capacitance for the supercapacitor since the electrode is notdeteriorated after fifty cycles.

Experiment 4

After an activated carbon powder was added to 60 ml of deionized water,the activated carbon powder was sufficiently dispersed and wetted in thedeionized water by stirring the deionized water, including the activatedcarbon, to form an activated carbon solution. The activated carbon has aspecific surface area of 1500 m²/g. In this case, the amount of theactivated carbon was 20 wt %, 40 wt %, 60 wt % and 80 wt %,respectively. The amount of the activated carbon was evaluated ongrounds of the total amount of the manganese oxide powder produced bymixing the potassium permanganate with manganese acetate during thesubsequent steps.

Subsequently, a permanganate solution was prepared by adding 1.58 g ofpotassium permanganate to the activated carbon solution. Thepermanganate solution was stirred for 1 hour so that the potassiumpermanganate was sufficiently absorbed on the activated carbon. When thepotassium permanganate was added to the activated carbon solution, thecolor of the permanganate solution was deep violet. However, the colorof the permanganate solution became transparent when the permanganatesolution was stirred since the potassium permanganate was absorbed inpores of the activated carbon and on the surface of the activated carbonhaving strong absorbability.

In the meantime, manganese acetate solution was prepared by adding 3.68g of manganese acetate to 100 ml of deionized water.

The manganese acetate solution was mixed with the permanganate solutionto form a mixed solution, and then the mixed solution was violentlystirred. When the manganese acetate solution is mixed with thepermanganate solution, a reaction rapidly proceeds with the formation ofamorphous manganese oxide, so the color of the mixed solution quicklychanges to brown, which is the color of the amorphous manganese oxide,and the viscosity of the mixed solution rapidly increases. Thus, themixed solution was stirred for 12 hours in order to sufficiently executethe formation reaction of the amorphous manganese oxide in the presentexperiment.

After the mixed solution including the amorphous manganese oxide wasfiltered several times with a ceramic filter, thereby obtaining anamorphous manganese oxide powder, the amorphous manganese oxide powderwas washed with deionized water.

Then, the amorphous manganese oxide powder was sufficiently dried in adryer at a temperature of 120° C. After the sufficiently dried amorphousmanganese oxide powder was ground, a mixture was formed by mixing 5 wt %of binder with the ground amorphous manganese oxide powder. In thiscase, the binder was PTFE. Subsequently, sheet-shaped electrodes werecut to form pellet-shaped electrodes after the sheet-shaped electrodeswere formed by rolling the mixture. Finally, manganese oxide electrodesfor a supercapacitor were manufactured by cold drawing the pellet shapedelectrodes on current collectors.

Experiment 5

In the present experiment, electrodes for the supercapacitor weremanufactured by the method of experiment 4 except for the type ofactivated carbon and the amount of the activated carbon in theelectrodes for the supercapacitor.

According to the present experiment, the activated carbon had a specificsurface area of 2000 m²/g and the amount of the activated carbon in theelectrodes was 20 wt %, 40 wt %, 60 wt % and 80 wt %, respectively.

The amorphous manganese oxide of the present experiment was analyzed byX-ray diffraction analysis so as to evaluate the characteristics of theamorphous manganese oxide.

FIG. 12 is a graph showing the X-ray diffraction analysis of theamorphous manganese oxide of the present experiment. The amorphousmanganese oxide was manufactured by adding 40 wt % of the activatedcarbon.

Referring to FIG. 12, the amorphous manganese oxide of the presentexperiment is nearly entirely in the amorphous phase in comparison withconventional manganese oxide, even though the amorphous manganese oxidehas a little crystal phase material.

In general, the performance test for the electrode is accomplished in abeaker-type electrochemical cell having a working electrode, a platinumgauze and a standard calomel reference electrode (SCE) therein. Thesurface area of the working electrode is about 0.25 m², and a 2Mpotassium chloride (KCl) solution having a hydrogen ion exponent ofabout 6.7 is used as electrolyte.

FIG. 13 is a graph illustrating cyclic voltammogram of the electrode forthe supercapacitor according to the present experiment. In FIG. 13, thecyclic voltammogram of the electrode is measured at a voltage scanningspeed of 20 mV/sec, and the electrode includes 40 wt % of activatedcarbon.

As shown in FIG. 13, the electrode for the supercapacitor of the presentexperiment has high current response and the charging/discharging of theelectrode can be accomplished by a constant rate concerning the wholecycle.

FIG. 14A to FIG. 14F are graphs showing cyclic voltammograms of theelectrode of the present experiment. In FIGS. 14A to 14F, the cyclicvoltammograms of the electrode for the supercapacitor were measured bychanging voltage scanning speeds and the electrode includes 40% byweight of activated carbon.

Referring to FIGS. 14A to 14F, the electrode of the present experimentshows the best property for the supercapacitor when the voltage scanningspeed is 20 mV/sec.

FIG. 15 is a graph illustrating variation of specific capacitance of theelectrode of the present experiment. In this case, the specificcapacitance is measured by changing the voltage scanning speed and thespecific capacitance means standardized value obtained by dividing ameasured capacitance by weights of the manganese oxide coated on theactivated carbon.

As shown in FIG. 15, the specific capacitance of the electrode decreasesas the voltage scanning speed increases. In particular, the specificcapacitance of the electrode is more than approximately 300 F/g when thevoltage scanning speed is 10 to 20 mV/sec.

Experiment 6

As for the present experiment, electrodes for the supercapacitor weremanufactured by the method of experiment 4 except for the type ofactivated carbon and the amount of the activated carbon in theelectrodes for the supercapacitor.

In the present experiment, the activated carbon had a specific surfacearea of 2500 m²/g and the amount of the activated carbon in theelectrodes was 20 wt %, 40 wt %, 60 wt % and 80 wt %, respectively.

Experiment 7

In the present experiment, electrodes for the supercapacitor weremanufactured by the method of experiment 4 except for the type ofactivated carbon and the amount of activated carbon in the electrodesfor the supercapacitor.

According to the present experiment, the activated carbon had a specificsurface area of 3000 m²/g, and the amount of the activated carbon in theelectrodes was 20 wt %, 40 wt %, 60 wt % and 80 wt %, respectively.

FIG. 16 is a graph showing specific capacitances of the electrodes forthe supercapacitor according to experiments 4, 5, 6 and 7. In this case,the electrodes respectively comprise 40 wt % of activated carbon.

Referring to FIG. 16, the electrode including the activated carbonhaving the specific surface area of 2000 m²/g shows the largest specificcapacitance according to experiment 5.

FIG. 17 is a graph showing specific capacitances of electrodes havingdifferent amount of activated carbon according to experiments 4, 5, 6and 7. At that time, the electrodes respectively comprise 20 wt %, 40 wt%, 60 wt % and 80 wt % of activated carbon.

Referring to FIG. 17, the electrode including 40 wt % of activatedcarbon having the specific surface area of 2000 m²/g shows the bestspecific capacitance.

Experiment 8

After potassium permanganate powder was formed by grinding potassiumpermanganate to have fine particles, the potassium permanganate powderwas inserted into a furnace previously heated to a temperature of 500°C. Then, the potassium permanganate powder was thermally decomposed for2 hours, thereby obtaining a manganese oxide powder.

After the manganese oxide powder was rapidly cooled (that is, quenched)below room temperature by using distilled water, it was washed withdistilled water, filtered and dried in a dryer.

The dried manganese oxide powder was mixed with conductive carbon andbinder to form slurry for an electrode. In this case, the slurrycomprises 90 wt % of the manganese oxide powder, 5 wt % of theconductive carbon and 5 wt % of the binder.

Then, the electrode for the supercapacitor was manufactured by coatingthe slurry on a current collector.

Experiment 9

As for the present experiment, the electrode for the supercapacitor wasmanufactured by the method of experiment 8 except for the composition ofthe electrode. In the present experiment, the electrode comprises 80 wt% of manganese oxide powder, 15 wt % of conductive carbon and 5 wt % ofbinder.

Experiment 10

According to the present experiment, the electrode for thesupercapacitor was manufactured by the method of experiment 8 except forthe composition of the electrode. In the present experiment, theelectrode comprises 70 wt % of manganese oxide powder, 25 wt % ofconductive carbon and 5 wt % of binder.

Experiment 11

In the present experiment, the electrode for the supercapacitor wasmanufactured by the method of experiment 8 except for the composition ofthe electrode. According to the present experiment, the electrodecomprises 60 wt % of manganese oxide powder, 35 wt % of conductivecarbon and 5 wt % of binder.

Experiment 12

In the present experiment, the electrode for the supercapacitor wasobtained by the method of the experiment 8 except for the composition ofthe electrode. According to the present experiment, the electrodecomprises 50 wt % of manganese oxide powder, 45 wt % of conductivecarbon and 5 wt % of binder.

Experiment 13

As for the present experiment, the electrode for the supercapacitor wasobtained by the method of experiment 8 except for the composition of theelectrode. According to the present experiment, the electrode comprises60 wt % of manganese oxide powder, 30 wt % of conductive carbon and 10wt % of binder.

Experiment 14

According to the present experiment, the electrode for thesupercapacitor was obtained by the method of experiment 8 except for thecomposition of the electrode. In the present experiment, the electrodecomprises 60 wt % of manganese oxide powder, 25 wt % of conductivecarbon and 15 wt % of binder.

Experiment 15

In the present experiment, the electrode for the supercapacitor wasobtained by the method of experiment 8 except for the composition of theelectrode. According to the present experiment, the electrode comprises60 wt % of manganese oxide powder, 20 wt % of conductive carbon and 20wt % of binder.

Experiment 16

As for the present experiment, the electrode for the supercapacitor wasobtained by the method of experiment 8 except for the composition of theelectrode. According to the present experiment, the electrode comprises50 wt % of manganese oxide powder, 20 wt % of conductive carbon and 30wt % of binder.

Experiment 17

In the present experiment, the electrode for the supercapacitor wasobtained by the method of experiment 8 except for the composition of theelectrode. According to the present experiment, the electrode comprises30 wt % of manganese oxide powder, 20 wt % of conductive carbon and 50wt % of binder.

Evaluation of the Electrodes Manufactured According to Experiments 8 to17

The capacitances and the ESR of the electrodes according to experiments8 to 17 are measured as shown in Table 1. In Table 1, the ESR ismeasured concerning capacitors of 2V and 500 mF.

TABLE 1 ESR of 2V, Experiment specific capacitance (F/g) 500 mFcapacitor (Ω) Experiment 8 264.2 1.768 Experiment 9 294.3 0.780Experiment 10 302.9 0.452 Experiment 11 311.1 0.329 Experiment 12 326.40.204 Experiment 13 327.5 0.174 Experiment 14 312.8 0.128 Experiment 15331.4 0.121 Experiment 16 294.6 0.153 Experiment 17 231.3 12.433

FIG. 18 is a graph showing an alternating current impedance of a 2Vcapacitor manufactured by using the electrode of experiment 15, and FIG.19 is a graph showing the CV of the 2V capacitor in FIG. 18. In thiscase, the capacitor has a capacitance of 500 mF.

Referring to FIGS. 18 and 19, the electrodes of the present inventionhave excellent properties for the supercapacitor.

FIG. 20 is a graph illustrating a performance test for the electrode ofexperiment 15 through continuous tests repeated twice and twenty timesso as to evaluate the cyclability of the electrode. In FIG. 20, dottedand solid lines, respectively, represent the continuous tests repeatedtwice and twenty times.

As shown in FIG. 20, the electrode of experiment 15 maintains goodcharacteristics for the supercapacitor after the continuous tests wererepeated twenty times.

FIG. 21A to FIG. 21D are graphs showing the cyclic voltammograms of theelectrode of experiment 15. The cyclic voltammograms were measured bychanging the voltage scanning speed.

Referring to FIGS. 21A to 21D, the capacitance of the electrode ofexperiment 15 may not be affected by the voltage scanning speed, and theelectrode has the best capacitance when the voltage scanning speed is100 mV/sec.

According to the present invention, the loading amount of manganeseoxide on the conductive carbon greatly increases and the degree ofdispersion of the manganese oxide is much enhanced in comparison withthe physically mixed manganese oxide with conductive carbon. Therefore,the electrode for the supercapacitor of the present invention can reducethe ESR and enhance the high frequency characteristics because thecontact area and the adhesion strength between the manganese oxide andthe conductive carbon are improved.

Also, the electrode of the present invention has a high capacitancesuitable for a supercapacitor as compared with a conventional electrodebecause the electrode is manufactured by mixing amorphous manganeseoxide powder with conductive material and binder.

Furthermore, the electrode of the present invention can be manufacturedat a cost of one percent of that of the ruthenium oxide electrode, eventhough the electrode of the present invention has good capacitance whichis nearly equal to half the capacitance of the ruthenium oxideelectrode, which has the highest capacitance among conventionalelectrodes.

Although the preferred embodiments of the invention have been described,it is understood that the present invention should not be limited tothose preferred embodiments, but various changes and modifications canbe made by one skilled in the art within the spirit and scope of theinvention as hereinafter claimed.

What is claimed is:
 1. A metal oxide electrode for a supercapacitorcomprising: a manganese oxide powder including potassium (K) ions; aconductive material; and a binder for binding said manganese oxidepowder to said conductive material.
 2. The metal oxide electrode for asupercapacitor as claimed in claim 1, wherein said conductive materialis composed of a material selected from the group consisting of a metaloxide, a metal nitride, a metal carbide, a metal powder and a conductingpolymer and said manganese oxide powder is coated on said conductivematerial.
 3. The metal oxide electrode for a supercapacitor as claimedin claim 1, wherein said conductive material is a conductive carbon andsaid manganese oxide powder is coated on said conductive carbon.
 4. Themetal oxide electrode for a supercapacitor as claimed in claim 3,wherein said electrode comprises about 20 to about 80% by weight of saidconductive carbon.
 5. The metal oxide electrode for a supercapacitor asclaimed in claim 1, wherein said conductive material is an activatedcarbon and said manganese oxide powder is coated in pores of saidactivated carbon and on a surface of said activated carbon.
 6. The metaloxide electrode for a supercapacitor as claimed in claim 5, wherein saidelectrode comprises about 20 to about 80% by weight of said activatedcarbon.
 7. The metal oxide electrode for a supercapacitor as claimed inclaim 5, wherein said activated carbon has a specific surface area ofabout 1500 to 3000 m²/g.
 8. The metal oxide electrode for asupercapacitor as claimed in claim 1, wherein said conductive materialis composed of a material selected from the group consisting of a metaloxide, a metal nitride, a metal carbide, a metal powder and a conductingpolymer and said manganese oxide powder has a layer structure.
 9. Themetal oxide electrode for a supercapacitor as claimed in claim 1,wherein said conductive material is a conductive carbon and saidmanganese oxide powder has a layer structure.
 10. The metal oxideelectrode for a supercapacitor as claimed in claim 9, wherein saidelectrode comprises about 30 to about 90% by weight of said manganeseoxide powder, about 5 to about 20% by weight of said conductive carbonand about 5 to 50% by weight of said binder.
 11. The metal oxideelectrode for a supercapacitor as claimed in claim 1, wherein saidbinder is composed of a polytetrafluoroethylene.
 12. A metal oxideelectrode for a supercapacitor comprising: a conductive carbon; anamorphous manganese oxide powder coated on said conductive carbon,wherein said manganese oxide powder is formed by mixing a first solutionhaving a potassium permanganate absorbed on said conductive carbon aftersaid conductive carbon is dispersed in the first solution with a secondsolution of a manganese acetate; and a binder for binding said manganeseoxide powder to said conductive carbon.
 13. The metal oxide electrodefor a supercapacitor as claimed in claim 12, wherein said electrodecomprises about 20 to about 80% by weight of said conductive carbon. 14.A metal oxide electrode for a supercapacitor comprising: an activatedcarbon; an amorphous manganese oxide powder coated in pores of saidactivated carbon and on a surface of said activated carbon, wherein saidmanganese oxide powder is formed by mixing a first solution having apotassium permanganate absorbed in the pores of said activated carbonand on the surface of said activated carbon after said activated carbonis dispersed in the first solution with a second solution of a manganeseacetate; and a binder for binding said manganese oxide powder to saidactivated carbon.
 15. The metal oxide electrode for a supercapacitor asclaimed in claim 14, wherein said electrode comprises about 20 to about80% by weight of said activated carbon.
 16. The metal oxide electrodefor a supercapacitor as claimed in claim 14, wherein said activatedcarbon has a specific area of about 1500 to about 3000 m²/g.